A conventional fixed venturi tube carburetor as shown in FIG. 8 includes an induction pipe I having a throat section V of a venturi tube for sucking fuel from a float chamber F through a choke tube C by an air stream A enterring the pipe I, wherein the fuel will not be completely atomized under slow running of an engine because the air flowing velocity is so slow as passing through such a fixed venturi tube of which the throat area can not be variably reduced corresponding to the engine slow running, thereby reducing the engine combustion efficiency, wasting energy and possibly causing air pollution due to incomplete combustion of fuel. Even at an idling situation, a throttle valve T can be closed to leave merely a narrow gap between the valve T and the carburetor wall to develop a suction effect as shown at A1 of FIG. 8 to suck fuel through a pilot system PS, which however still dissipates into two air streams A1 and A2 to thereby reduce the air velocity and reduce the suction force for the pilot system, resulting in a poor atomization of the fuel droplets and a lower engine efficiency accordingly.
A conventional variable choke carburetor is shown in FIG. 9 may even variate its choke area by reciprocatively moving a jet needle J within a nozzle N as actuated by a piston P in a suction chamber S for a better mixing of air A with a fuel from float chamber F. However, when the jet needle J is subject to a slow or idling engine running having small gap between the needle and a nozzle orifice, the needle J will be easily worn to influence its fuel suction capacity due to a frictional scrubbing between the needle and the nozzle orifice under frequent vibrations caused by the reciprocative movements of the piston P.
The present inventor has found the drawbacks of conventional carburetors and invented the present rotor-actuating carburetor having a variable venturi tube.